Engine knock is caused by spontaneous combustion of the air/fuel mixture in compressed end gasses caused by the initial combustion spark event in a combustion chamber. Knock sensors may be employed to identify knock conditions, such as due to carbon buildup inside the combustion chamber. The knock sensor may be a passive piezoelectric device that outputs a voltage in response to the received acoustic vibration. A high amplitude knock sensor output may indicate a knock event. In order to ensure engine performance, a rationality check on the knock sensor may be used.
Attempts to address knock sensor rationality checks include determining knock sensor degradation based on the sensor output during engine operation. One example approach is shown by Hernandez et al. in U.S. Pat. No. 7,222,607 B2. Therein, knock energy of each knock sensor is computed, and a knock sensor fault is determined if the knock energy is lower than an experimentally determined threshold.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain condition, the energy of the knock sensor output may be lower than a threshold even when the knock sensor is working properly. For example, during vehicle idle stop or an electric vehicle (EV) mode of a hybrid electric vehicle (HEV), the knock sensor output may be flat at nearly zero. Under these conditions, prior approaches may lead to false positive diagnosis. Further, rationality checks based on knock energy may be time consuming, since a reliable knock energy calculation may require large sampling points of the knock sensor output.
In one example, the issues described above may be addressed by a method for an engine, comprising: generating a vibration via an actuator in the absence of engine combustion; and indicating knock sensor degradation based on a knock sensor output responsive to the generated vibration. In this way, engine knock sensor degradation may be reliably determined.
As one example, a method may include recording an engine vibration during engine combustion, and generating an excitation signal by adding the recorded engine vibration with a pulse signal. When vehicle is still in operation and the combustion events in all engine cylinders are stopped, an actuator may be actuated with the excitation signal to generate a structural and/or an acoustic vibration. The vibration may simulate the sound or vibrations of engine knock during engine combustion, and trigger a knock sensor (e.g., where processed output of the knock signal would be identified as engine knock by the electronic control system). The vibration, which may include acoustic vibration, may also alert pedestrians and/or vehicle operators that the vehicle is running when there is no engine combustion noise. Based on the knock sensor response, sensor degradation may be indicated. For example, knock sensor degradation may be indicated if amplitude of the knock sensor output corresponding to the excitation pulse signal is lower than a threshold. In this way, a knock sensor rationality check may be reliably performed when there is no combustion event in the engine cylinder (e.g., when the engine is at rest and no combustion cycles are being carried out). Since knock sensor degradation is determined by checking if the sensor is responsive to a controller generated pulse signal in this example, the rationality check may be completed in a short time. In an example, by adjusting the amplitude of the acoustic vibration to within a threshold of the same level as sound generated from engine knock, knock sensor degradation may be determined reliably. Further, by comparing the amplitude of the excitation signal with the amplitude of knock sensor output, degradation in sensitivity of the knock sensor may be compensated. In an example, the actuator for generating the acoustic vibration may be integrated with an acoustic vehicle alerting system (AVAS) that already exists in the HEV. Therefore, the method may be implemented with little adjustment to the vehicle system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.